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Why are 3- or 4-seat gliders so rare?
Sailplanes capable of climbing in rising air (not troop transports, which can only descend after the towrope releases) have one or two seats.  I know of only one type with more, the Schweizer SGS 2-32 "2½ seater."  Even experimental one-offs like the Perlan series have at most two seats.
Ignoring social aspects such as marketing, class associations, and racing leagues, what are the technical difficulties in making/flying/maintaining a sailplane with more than two seats?  Frontal area, if seated abreast?  CG range, if seated fore-aft?  Something special about mass or wingspan that doesn't apply to powered airplanes?

There's nothing technical that can't be overcome, there simply is no market for them.

Who wants a family argument in a sailplane? "Are we there, yet, Dad?" "Mummy, I want to pee." The very thought is enough to give one the shudders.

@GdD Why can a dust speck fly, but an elephant can't? Of course there are also technical reasons.

I said there's nothing technical that cannot be overcome @Peter-ReinstateMonica. There were airplanes flying to the edge of space within 50 years of the first powered flight, if there was enough of a market for 4 seater gliders they would be there.

@GdD The literal meaning of what you said is true; but "cannot be overcome" stops only short before a hyperdrive, so in context it meant "there is no significant technical reason, which is probably not true. (I am a total layman and only arguing from first principles; perhaps you know how much harder or more expensive it would be to build a 4-seater? It's possible that only a >20-seater would be really hard while a 4-seater is totally no big deal. But I suspect that cost and constructive issues play a role.)

Same reason you don't see four people on a surfboard?  Or surfboards designed for same.

Yes, no significant technical problems, just lack of demand.  Soaring is something you do solo.  The only reason for even 2 seat sailplanes is instruction.  (OK, and maybe giving tourist rides.)  It's like bicycles: while there are "bicycles built for two" (or even more), they're hardly what the typical bike rider wants.

One possible difficulty, though it's more economic than technical, is that a 4-place sailplane is going to be heavier, and have more drag, than a 1 or 2 place one.  That means you need a more powerful (= more expensive to buy & operate) towplane.

Broadly because of weight-to-strength…

There are technical limitations.
When I fly a two-seater, my thermalling technique is different from flying 15m single seaters. With the single seater it is no problem to center the thermal while observing the variations in climb speed. Tighten the turn when climb speed drops and vice versa.
But with two-seaters I wait for a three quarter turn and only then apply my correction. That means I have to wait with a correction and have fewer chances for them. If I wanted to fly like with a single seater, my corrections would come too late and shift me further from the center of the thermal. Two seaters are heavier and have more wingspan, so both inertial moments and roll damping are greater and make all responses more sluggish, requiring me to fly corrections more deliberately.
Now extrapolate that to a three- or fourseater! While thermalling a single seater (Discus, ASW-24) is fun, doing so in a large two seater (ASH-25 or SB-10) is real work. I cannot imagine doing this successfully in a glider that is again a lot heavier.
The only gliders with more than two seats were designed for gliding only.

More detailed explanation:
After two or three turns in a thermal you have already a good mental picture where you are relative the core of the thermal. A single seater is agile enough to make corrections immediately, using the change in climb speed as a cue.
A two seater (and an open class glider) is more sluggish and requires to think more ahead. Now those immediate changes become counter-productive because they will be out of phase with what you intend. You have to wait your turn (literally) for the next change, so you will be slower to optimize the flight path.
On top of that: On weak and especially cloudless days thermals are narrow and the higher wing loading of the two seater forces you to fly wider circles, so you will stay in a weaker thermal than the single seater which can fly tighter circles. Going to more seats will drive up wing loading and inertias more and force you to fly even larger circles which give you even less climb speed.

Re last sentence-- the Schweizer 2-32 being a notable exception-- although, technically, I guess the two passengers in the rear are both sitting in the same "seat"...but by similar logic, there may have been only two "seats" in the glider in your link as well--

@ymb1 -- for whatever it's worth , my own technique is to wait till I've passed through the quadrant of strongest lift (say for example, the strongest lift appears to be in the northern quadrant), (sometimes waiting till I've seen the same indication for two circles in a row), then in the next circle at the point I'm heading straight north, straighten out and fly wings-level for about four seconds and then roll back in to resume the circle.  So I'm reducing bank angle (actually often all the way to level) 270 degrees after the heading (eastbound in this example) where I saw the strongest lift.

re "(eastbound in this example) "  -- that would be for a clockwise circle.

@quietflyer While you can squeeze two slim persons into the back seat of  a 2-32, the leading 2 in the name should make clear that it is intended to seat only two.

@ymb1 I expanded the answer. Please let me know if that helped.

I don't really see any support for the argument that there are technical, and not purely market, limitations. What is being described has more to do with experience and suitability to a particular region, which aren't really technical limitations. The Stemme motorglider is a great example of a glider which has all the characteristics you list and it is indeed not a great thermalling ship because the stall speed is higher than other gliders. But it excels on the mountain slopes and in wave. A 4 seater pure glider would weigh comparably and perform comparably. No one wants one is all.

@KennSebesta. Right. And why doesn't anybody want one? Because flying them is not much fun compared to a single seater. For the reasons listed above. Technical reasons.

I think we will have to agree to disagree on the definition of "technical". I haven't been able to correlate "not fun" with "technical limitation". Out of curiosity, what keeps me from using this definition of technical limitations as the explanation for why I can't find a two-person toilet seat at the hardware store?

Because much of the fun in being in a sailplane, is in actually flying the sailplane.  It's not a means to get from point A to point B, it's a means to enjoy the magic of flight, much of which involves seeing the sailplane respond to your control inputs.
Once you start getting more than two occupants, most of the occupants won't get to do much of the flying.
It's kind of like asking, "why aren't there more saddles for horses that can handle 3 or 4 people?"
As an aside, raw novice passengers tend to be at high risk for airsickness when subjected to prolonged thermal turns in turbulent conditions, especially if they get to do little or none of the actual flying.  Three or four non-flying passenger seats would potentially equate to a lot of clean-up afterwards.
I guess it's all a little subjective-- a commercial pilot friend of mine regularly gives 15-to-60 minute semi-aerobatic rides in a glider with room for two passengers squeezed in side-by-side on a seat in the back (Schweizer 2-32), with controls for the pilot only, and most of his passengers love it (and most don't get sick).  But being seated tucked in close to a friend for a little while feels different than riding on a "bus" with, say, four or more strangers-- and in that particular case the very same glider can have the second control stick re-installed in the back and be used for student instruction with one rear-seat occupant and one front-seat occupant.  A bigger "bus" with more ample seating for multiple passengers probably would be a poor candidate for a training glider.

You make a good point.  Being a passenger in a sailplane is a bit like riding on a hockey plaver's back. After a while, it gets boring and you either want to be the one working the puck or go do something else.  My wife likes flying but finds riding in circles in a glider to be boring.  It's the same problem with sailboats.  I've flown a 2-32. I thought it flew like my old Cessna Cardinal but with the engine missing and a much flatter glide.  The energy potential in its mass relative to other gliders made it a bit demanding for some pilots and only senior club members were allowed to solo it.

Not sure about the horse analogy.  You can build a bigger plane, but building a bigger horse is a bit more of a challenge.  Sure you can breed them bigger to a point, but we're not likely to ever see a horse with the seating capacity of a 747...

@DarrelHoffman -- true, it just sounded good at the time-- I'll leave this as one of my rare answers that does not get over-edited : )

"It's kind of like asking, "why aren't there more saddles for horses that can handle 3 or 4 people?""  that truly hits the nail on the head, nicely put.

@Fattie: Not really, because with horses you DO have serious technical limitations on the amount of weight they can carry.  (Rule of thumb is not more than 20% of the horse's weight.)  Even two people is not a good thing unless they're small people on a big horse.

heh - true enough @jamesqf .  perhaps the "surfboard" one someone mentioned is better.

As you suspected, the reasons boil down to performance and cost.  CG management can be handled by using a 1-2-1 seating arrangement, with little drag penalty (if the cockpit shape is optimized for this) compared to a tandem -- but when flying light, that would leave the passenger in the back seat and conversation in the cockpit would be difficult with the wind noise at high speed.
Overall, however, everything in a sailplane is generally minimized.  Dry mass (if you want to fly heavy, you fill ballast tanks with water), drag, and so forth.  At a minimum a cockpit design that keeps the CG position optimal will result in some extra front area, producing more drag, and the seats, even empty, will comprise mass that can't be dumped by opening a valve as would be the case with ballast added for speed.
Further, the main reason to want to carry more than two (pilot and copilot or instructor and student) is transport from point to point, which isn't, in general, what sailplanes are built to do.
There are motor gliders with side by side layout -- but they don't have the soaring performance of a proper sailplane due to the compromises that make them more like a light power plane than even a self-launched sailplane (larger engine, smaller wing, heavier construction, conventional gear rather than monowheel, usually) -- compromises which also mean losing the lift at low altitude doesn't mean an out landing, because you can just start the engine.

Apparently, I was incorrect -- side by side, yes, but four-seat appears to be out of the "touring motor glider" class.  I might have been remembering something from long ago, too, but it seems there are only about a dozen current types.

@ymb1 That thing was specifically designed to win NASA's Green Flight Challenge... I heard it wasn't really a joy to fly with

@ZeissIkon : Those "touring motor glider" are closer to ultralights (or even just small GA planes) than to pure gliders.

@vsz Not too many airplanes in the Cessna or Piper light plane class with 30:1 glide ratio, though.  They have some compromises -- max 2 seats, apparently, extra wingspan (which makes them harder to hangar), low horsepower (which limits level flight speed and climb rate) in order to have the glide performance they need to fly as sailplanes when the engine is off.

Most touring motor gliders typically can't reach a 30:1 glide ratio either.

Another concern is ground handling.  All ground operations for gliders tend to be pretty labor intensive.  Gliders can't taxi under power.  So, they are maneuvered using human muscle and tow vehicles.  They are launched while a person holds up the wing.  They taxi the same way.  Many can be (fairly) easily disassembled for storage or when they land out (in a field away from an airport).  Many can just be put into a trailer by pulling off the wings.  But, if two people can't carry each wing, it becomes much more difficult.
While each of these things could be designed around, they do add extra limitations on the design.

FWIW, I taxi an AC-5M all the time (retractable pylon-mounted engine). It just takes practice, patience, and not too much wind (or else the plane can't overcome the weather-vaning tendency. Works as well on grass as on pavement. It helps to inflate the tire a little more than standard. Also, make sure to try to keep even a little bit of speed so as to minimize scrubbing the tire. Also, pay attention to brake temperature, as with extended taxiing it's easy to overheat the tiny brake and cause brake fade.

Brake temperature gauge?  Divert some water ballast to cool the brake?

There are no technical limitations.
Only market forces.
Other answers point toward design issues, but I disagree that those are relevant to 3 or 4 vs 1 or 2, since they are the identical design issues which have to be met for any airplane. And since we have known for almost a century how to build 4-seaters that can fly at 200+kts, it's very easy to build one which only has to fly at 50kts.
Technical questions
Weight, frontal area, and C.G. are not issues:

the side-by-side Stemme S12 achieves 53:1 L/D and its MTOW is 1860lbs. This is one of the highest performance gliders currently available.
A good-sized tow plane can haul up heavily ballasted gliders with 1-3 people's worth of water stored in the wing.
Modern design techniques and materials allow side-by-side craft such as the Pipistrel Taurus to completely outperform the 1940's design Schweizer 2-33, which itself is still in daily use in American clubs.
Any regular four-seater can be flown solo, so there is no C.G. limitation which cannot be accounted for through time-tested design.

Practicality
There's never been any demand for a four seater glider. @quiet flyer has it exactly right, it's pretty boring to be in a sailplane and not be flying it. If you're flying the plane, it's mentally very engaging and thus the time flies by, but when you're a passenger you'd just be spending hours looking out the window before winding back up where you started.
So long as you're not looking to exceed, say, 30:1 L/D, it would be very easy to build an experimental 4-person glider. However, when the pilot is alone it would not perform anywhere near as well as a single-person glider, and there will only be a few times a year-- if that many-- that the pilot would have a plane full of people. So why build it and maintain it?
To take paying passengers would require regulatory certification, which would be very expensive. It would only make sense if the designer intended to sell enough to make back the development and certification costs. Again, market forces dominate here.

It's clear that the leisure market for 2-4 seater gliders is likely small but I wonder how much of a market there would be for more utilitarian uses, especially as fossil fuels will be increasing in price with the necessity to combat climate drift. For relatively short distances in mountainous terrain, gliders may turn out to be a relatively useful passenger transport mechanism as long as transport duration is not an issue.


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What is trimming the fuel tank?
In a recent news article on an Airbus A400M crash, it has the following line:

Sources have told Aviation Week that aircraft MSN23, destined for Turkey, featured new software that would trim the fuel tanks, allowing the aircraft to fly certain military maneuvers.

I understand what trimming is, but what does it mean to trim the fuel tanks?

Just like normal trimming. However trim the fuel tank use aft fuel tank to achieve the trimming effect. Fuel is pumped between main tank and aft tank to shift CG around to a desire position during flight.
Fuel tank trimming has additional benefit of keeping the airplane in "clean" configuration, creating less drag compare to normal trimming.

What is "clean" configuration of an airplane?

@iamcreasy: Gears and flaps up; read "clean" as "aerodynamically clean".

In this context, clean refers NOT to gear/flaps, but to the absence of drag due to trim tabs or drag-generating control forces keeping the aircraft level.  In concept, if the tail has to produce lift down because the aircraft is nose-heavy, that's less efficient than if it didn't have to.

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Who is the pilot in command when the pilot in command is resting?
On long-haul flights, there are extra pilots on board so that the flight crew gets a chance to rest. When the pilot in command gets relieved and goes to the rest area, who is legally in charge of the airplane? Is the PIC role transferred to one of the pilots in the cockpit, or is the person who was relieved still ultimately in command? (I'm especially interested in what happens if the relieved PIC feels that the crew is mishandling the aircraft and wants to come back and take control).

If a situation were to happen where a professional captain felt that she needed to commandeer an aircraft from another professional because of incompetence, someone would probably get fired or arrested upon landing.  I don't know what there is any precedence or official rule for this situation.  I think common sense would prevail.

http://aviation.stackexchange.com/a/31429/679 Faa suggest PIC can be changed during flight. I guess the rest depends on each airlines' SOP.

I know that on AF447 the captain went on rest break and assigned the FO that was in the right seat as PIC even though the FO that relieved the captain had more experience. But when they called the captain back to the flight deck they deferred to his authority.

@TomMcW: The way this transfer of PIC responsibility was done has been a contributing factor. See page 169 "2.1.1.3 Relief of the Captain".

I understand that (a) the PIC is ultimately responsible for the whole flight, including the planning, the route, etc. This responsibility cannot be transferred. (b) If the PIC leaves the cockpit for a rest, he needs to appoint a qualified substitute. The substitute will temporarily act as PIC, but does not become the PIC.

It might be beneficial to specify a jurisdiction here. I'm not sure that this varies much by jurisdiction, but I also wouldn't be surprised if the details are different in some different jurisdictions.

See also How is aircraft control positively transferred between pilot flying and pilot not-flying?

The PIC is ultimately responsible for operation and safety, for everybody and everything on board, from pre-flight preparation till post-flight activities.
On long-haul flights there must be a sufficient number of relief crew members to substitute the nominal flight crew. This is regulated in national legislation and implemented in airline standard operation procedures (SOP). Details are apparently different from airline to airline.
The Captain is the highest ranking member of the flight crew, and will be designated as PIC. When the PIC takes a rest, there must be a qualified substitute available. That will be a Senior First Officer (SFO) in some airlines. In any case, the substitute will have gone through a dedicated training.
During the rest of the PIC, the substitute will act as PIC. The sleeping Captain remains PIC, and remains ultimately responsible. If there is an incident during his rest, it would have to be investigated if that incident was (partially) due to earlier decisions, actions or omissions, or if the problem was caused entirely by the relief crew.

Ultimately that the PIC (e.g. the Captain) is responsible for the entire flight. But responsibilities that form that role can be delegated (e.g. to the First Officer) as in the case of the crew member leaving the cockpit. Pilots operator in a type of no-blame culture. One pilot wouldn't be held accountable for another's action unless they knowingly encouraged or instructed them incorrectly whether they were sitting next to them or not.
Even if both crew are in the cockpit, the First Officer is encouraged to take action if they believe the Captain is making a poor decision. There have been accidents over the years where a Captain ruled whilst the First Officer sat there quietly. So, many years ago a cultural change was pushed to help reduce accidents where the outcome was avoidable if the other crew members spoke up.

Welcome! While the first part is a good illustration of the climate, the answer doesn't tackle yet the legal aspect the OP would like to clarify.

There is no instructions that I can see that the PIC is legally responsible for the malicious intents of anyone else. If the actions were at the direct instruction of the PIC (or company), then consequences would fall back on the PIC (or company). The PIC would be legally responsible for the safe operation of the flight and if someone else contradicted that, then that's their problem. The same scenario would apply in any field.

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Is the GPS ground track true or magnetic?
This question arises from the Garmin GPS units (eg. GNS530 or GTN750), but the term ground track appears in a general context of GPS units so my question is also general.
Is the ground track (TRK in Garmin) a magnetic or a true course or does it depend on a specific GPS unit?
Intuitively, ground track should be a true course as it is (presumably) computed from GPS coordinates over time. But I can't find an authoritative (or any) source discussing how the ground track is computed in GPS units and is it true or magnetic. I'm specifically interested in Garmin units, but any information will be useful.

"Intuitively, ground track should be a true course as it is (presumably) computed from GPS coordinates over time. " -- just keep in mind that these units no doubt have a stored world map of magnetic variation, so it would be a simple matter to "correct" from True to Magnetic or vice versa.

It's magnetic, take a look at this answer (for a different question) which contains the AIM quote describing how it is calculated, and why it is common to see small differences between GPS and charter courses:  https://aviation.stackexchange.com/a/97757/69

Is the ground track (TRK in Garmin) a magnetic or a true course or
does it depend on a specific GPS unit?

In some Garmin non-aviation-specific handheld GPS units (e.g. 76S, and most of the Etrex series) the function you are speaking of is called "Heading" rather than "TRK".  Since these units lack magnetic sensors, it is (despite the name) computed entirely based on the very recent (i.e. near-instantaneous) ground track.  The user may select whether the displayed value is a Magnetic value or True value.
In related variants that also include magnetic sensors (e.g. Etrex Legend), the user may switch off the magnetic sensor entirely, or may select a minimum groundspeed value below which the "Heading" value will be based on on the magnetic sensor rather than the ground track.1  If the magnetic sensor has not been switched off, above the selected groundspeed value the "Heading" value will be based on the ground track.  The logic of this design is clearly that with a handheld unit, optimized for use while hiking etc and not intended to be used to infer wind (or current) correction angles, above a certain groundspeed, track-based information is more reliable than magnetic-based information.2  In such cases, the user's selection of Magnetic or True information for the "Heading" value will apply regardless of which of the two modes (ground track-based or magnetic sensor-based) the unit is operating in.
My experience has shown that the units mentioned here, while not designed specifically for aviation, can actually be extremely useful in various non-IFR aviation applications, including cross-country navigation over a cloud deck, and soaring.  All the units mentioned here have the ability to display current glide ratio and glide ratio to destination, which is very handy for soaring.  Wind direction can be observed (and wind speed can be estimated) by the direction of drift shown in the plotted ground track while thermalling (e.g. while flying in multiple circles), or alternatively by taking a moment to find the "Heading" that yields the lowest groundspeed.  A nice feature of the 76S in particular is its ability to display up to 12 numerical data fields on the same page as the map display.  For soaring in particular, it is often convenient to mount the unit on a small board that straps to the pilot's leg.  Just be certain that if the particular unit does have a magnetic sensor, it is switched off-- for the reasons given in footnote 2, a simple 1-axis magnetic sensor is extremely impractical to use in flight.  The aircraft's wet magnetic compass, or gyroscopic heading indicator (if present), will serve much better if the pilot wishes to compare the aircraft's ground track with the aircraft's actual heading.  The magnetic heading sensing function on dedicated aviation GPS units is obviously based on more sophisticated sensors and algorithms than are present in these simple hand-held units.
Footnotes:

"Groundspeed" is my term; these units just use the term "Speed".

Note that unless held perfectly level, the simple 1-axis magnetic sensor in these units suffers from "tilt" errors that are very similar to those experienced by a conventional "wet" aviation compass.  In actual practice these make the magnetic-sensor-based "Heading" function even more difficult to interpret than a conventional "wet" compass during banked, turning flight.

The Garmin units display their ground track as magnetic heading.
Source: We recently performed a test-flight with in which we compared the Garmin display to our experimental test recording devices (including GPS and INS) because our test pilots pointed out that some discrepancies between the Garmin "Track" and the true course heading which we calculated. However, I should point out that we could not find an according mention in the handbook of the Garmin device.
That Garmin chose magnetic Heading as the "Track" angle makes a bit more sense if you think about how pilots fly: If they speak about heading they pretty much always mean magnetic heading, because that is what their instruments display. If Garmin would choose to display the true heading, Pilots would be very confused, why their heading deviates by a couple of degrees from what their instruments would display...

The GPS is probably calculating a great circle route, which is generally NOT a constant true (nor magnetic) course... the no-wind heading needed to maintain that GC route will change along the route. So if you've computed "the" TC between 2 points, it's to be expected that the GPS won't always agree, especially over long distances.

@mins Yes, not the entire great circle, but the salient point that its heading changes along the track remains.

I'm not a pilot other than taking ground school many years ago, but I thought ground track and course are tied together, i.e. they are referring to the actual travel over the ground based on true north. Meanwhile heading is which way the nose is pointed and normally based on magnetic.  For example if flying between two airports that are at the same longitude the ground track would be north, although due to crosswind the heading might be slightly northeast. I'm wondering why a pilot would be confused if the GPS showed their ground track as 360 instead of a magnetic number.

For aviation applications, for reasons given in the your answer, in most cases using "magnetic" headings makes the most sense-- yet in some applications-- specifically navigation very near the magnetic poles -- it seems that "True" headings might be much more useful. I'd be surprised if dedicated aviation units gave the pilot no option to switch to "True", but can't speak from experience. –

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Were propeller airplanes significantly more "scary" to fly in compared to modern jet ones?
I've flown exactly once in my life. It was in 2004, with a commercial jet. Not a "jumbo", but a normal-sized jet plane in common use for cheap flights in that year. (Well, twice if you consider the trip back home as well, with a similar or identical plane.)
I found the experience quite scary. especially as it was going up and up, seemingly turning off the engines every now and then. The plane was "creaking" in a scary manner and it felt like the entire "main body" of the aircraft would snap in half at any moment, or that it would go too steeply upward and thus "turn around" and fall down on the ground or the sea.
Still, I wasn't hysterical and making a scene or anything. It just felt really scary in my stomach, perhaps understandably so since I had not grown up flying like most people seem to have. I was 18 when this experience happened.
I've many times watched old movies and documentaries and read about old airplanes, the "classic" kind which used propellers instead of the jet engines. I also assume that there must be multiple very different "generations" of jet-based airplanes as well, each one becoming more and more "comfortable" and "less scary" to travel in. Is this a correct assumption?
My question is: did old airplanes, for example ones in commercial use in the 1970s or 1950s, shake and creak more than modern ones, which would make them feel "scarier" to somebody like myself? I have this idea, which may or may not be accurate, that a jet engine is more "even" and that it perhaps has more "strength" to go through stormy weather and whatnot in a less "jerky" manner.
But on the other hand, older airplanes seemed to have way more room, less people and way better service, and not built with cost-cutting due to "ultra-cheap" tickets (I still find them expensive for my wallet), so maybe the overall experience was still more comfortable in those days?
I'm wondering about both the technical facts as well as the general perceived experience.

Comments are not for extended discussion; this conversation has been moved to chat.

Old propeller airliners were much more prone to "bumpy" ride than modern jets, for one major reason: they flew lower.
When I was in my early teens (almost fifty years ago), I flew several times aboard a Beechcraft 99, a low-wing, twin turboprop feeder aircraft with (IIRC) 14 or 18 seats.  Unpressurized, and most if not all seats have a pretty good forward view out the aircrew's windshield, as well as to the side over the (IIRC) right wing.
This aircraft had a cruise speed of somewhere around 200 kt, and was unpressurized, so flew below 12,000 feet (3500 m).  On a sunny day, there would be a bump every time the aircraft passed through a thermal column; if there was weather, every gust was palpable.  During takeoff and especially landing, the wing outboard of the engine flexed visibly, and the noise from the propellers was enough to make conversation difficult (not to mention anyone you might talk to was seated either ahead of you or behind).
I didn't find this scary, because I was already an aviation enthusiast; I found it fascinating and exciting.
But now, consider that the Beechcraft 99 was a significant step up from a slightly larger aircraft of a few decades earlier, the DC-3.  It had more power, quieter engines, and flew faster (not to mention smelled better -- at least to me, jet fuel kerosene is much more pleasant than leaded gasoline).  It climbed faster and had more reserve in case of a go-around or engine-out, as well.  Yet, the DC-3 was, when introduced, one of the highest performing transport aircraft in the world.
Every generation of aircraft has been an evolutionary improvement over the previous -- because that's why it replaced the one before.  Even the smallest jet airliners of today are much larger, more robust, faster, and because they fly higher, smoother.  The wings still flex -- as has every wing ever built since the Wright brothers got an engine -- and when you're low and slow, you'll feel bumps from turbulence and thermals, but you'll feel them less than you would have fifty years ago.

Isn't another reason that small planes are more susceptible to turbulence, and commercial planes of the pre-jet era were typically smaller? So the motor type is a red herring: A B747-sized propeller plane flying at 30,000 ft would fly as smoothly (but I think their ceiling is lower, so they cannot avoid weather systems by climbing higher like a jet).

@Peter-ReinstateMonica The ability to pressurize large propeller planes so they could fly higher came at the very end of their reign -- the Super Constellation was pressurized and could cruise high enough to be effectively turbulence free (and had similar passenger load to a first-gen 707) -- but by the time they fixed the fatigue problem, the 707 was in the market and DC-8 was nearly ready to sell.  As I said, it's mostly about altitude (and to a lesser extent wing loading, relative to turbulence -- higher loading makes the airplane respond less to a bump).

Re "jet fuel kerosene is much more pleasant than leaded gasoline", I think I would worry a bit if I smelled either - except when I'm actually fueling my own plane, of course :-)

Last paragraph is true for other vehicles as well.  Ever try driving a car from the 1950's or earlier?  By modern standards it's a pretty harrowing experience.  Weak brakes, sluggish (non-power) steering, hardly any shocks, no seatbelt, airbags, or any other safety equipment, you'll feel every bump in the road, and be constantly afraid you're going to lose control of the vehicle.  Technology improvements make lots of things less scary...

@jamesqf There's always a trace of fuel odor in the engine exhaust, and tiny planes like the Beech 99 were boarded directly from the tarmac, with engines idling in most cases, so you'd get a little of that smell.

@Zeiss Ikon: Perhaps your nose is better than mine, since I only smell gas when fueling, or if I get a bit on my hands or some sticks to the fuel tester.   Haven't any personal experience of jet fuel, though.

@Darrel Hoffman: With any decent car, you are going to get a good bit of road feel.  Some of us don't regard "handles like a waterbed" as an improvement :-)  But I guess it's subjective: I had quite a lot of fun in some '50s cars. and didn't regard them as scary at all, just as I don't think my '60s Cherokee is scary.

@jamesqf I debated where exactly to draw that cut-off.  Maybe pre-'40's?  '30's?  Agreed that older cars can certainly be fun to drive - there's a lot of overlap between "fun" and "scary" though.

My first car was a '63 Galaxie, pretty "boat" handling.  My second was a 1970 Rover 2000TC, much more "sporty" despite being a 4-door sedan.  Even in the 1950s, there were "sportier" cars (Studebaker Hawk, Corvette) and less sporty ones (Ford Fairlane and Galaxie, Cadillac anything, etc.).

@Zeiss Ikon: Depends on what cars you're looking at.  I certainly wouldn't want to drive the '59 Chevy that was one of the family cars when I was learning to drive - heck, I really would have preferred something else even then :-)  But like most American cars of the era, it was a far cry from the Austin-Healey, Sunbeam Alpine, and others of the era that I drove once I could afford my own car,

Small propeller planes are still used in rural areas. Sometimes the airport will provide passengers with ear plugs, and flying low over mountains in the spring is as bumpy as it is beautiful!

Yes, if you were riding on a DC-7 or Lockheed Constellation, the planes that the first passenger jets replaced, you would be crawling up the walls if you found a modern jet frightening.
Lots of noise and vibration from 4 Wright R-3350 radial engines, that belch great clouds of oil smoke when they start (to me, all the noise and smoke is a symphony, but not to most people I'm sure).
It's noisiest near the front where the propellers are, so the 1st class section was usually at the quieter back of the cabin instead of the front. In-flight engine shutdowns were relatively common with the complex turbo-compound version of the R-3350.
Vibration transmitted into the cabin from the engines will make the interior panels rattle and buzz (still a problem on turboprops today actually).
Service ceilings are in the area of 25000 feet, so you have to go through or around weather, not over it, so you may be in turbulence and icing during the cruise phase, not just during the brief departure/climb and descent/approach.
This movie, shown in theatres to promote air travel  and hosted by Arthur Godfrey (an early 50s kind of Oprah Winfrey type tv host) captures what it was like pretty well.
Of course, unless you were upper middle class or higher, you would be going by train or ship anyway, pre jet age.  Air fares were too expensive for regular people.

Ah, the wonderful Constellation. Experiencing in-flight engine shut-downs so often it was lovingly referred to by its pilots as "the best three engine airplane in the skies".

To enlarge upon John K's answer:
At night, the exhaust stacks (visible through the windows!) on the DC-7C glowed red-hot and the exhaust gas itself glowed blue and pink as it flowed back along the engine nacelle, flickering all the way to the trailing edge of the wing. This scared the heck out of me in 1960 while flying from LAX to Copenhagen via Winnipeg and Soendre Stromfjord.
In the daytime, the propeller de-icing systems would throw ice particles off the blades. Since the restrooms were positioned in the plane of the propellers (to minimize loss of life if a blade came loose and got thrown through the fuselage) and the restrooms had a little window in them, those ice particles would strike the window and produce little puffs of ice crystals that you could see.
If the props were even slightly out of sync, ripples of vibrations would rhythmically sweep through the passenger cabin and a cup of water placed on the fold-down tray table in front of you would "walk" by itself to the edge of the table under the influence of the vibrations and fall to the floor.
All these things made flying around the world in a piston-engined prop plane a lot scarier than the same flight in a jet.
And to top it off: because you flew lower and slower, the plane was constantly heaving and bucking up and down. After 23 hours of this, I suffered from severe mal-de-debarquement syndrome and for the following 10 years, every time I got into a confined space like a clothes closet the floor would start bucking and heaving and I would get nauseated!

A decently modern turboprop is comparable to a modern jet powered plane. Not much difference in the passenger experience, apart from the noisier engines.
However, a small propellor powered plane like a Cessna is a totally different beast. I have only flown on those a few times and they are like being on a bicycle in the air. A jet powered commercial airliner feels like a large luxury sedan in comparison.
Everything on those tiny planes is rattling & shaking. Safety precautions are minimal at best. I would imagine older smaller prop powered planes would be similar.

The first time I flew on a small plane was in a co-worker's Mooney. A screw dropped into my lap during the flight. That wasn't very comforting.

Great subjective description of the experience, but “Safety precautions are minimal at best.” isn’t so accurate — any plane produced by an established manufacturer in the last 50 years has plenty of well-designed and regulated safety features and oversight.

@PeterLeFanuLumsdaine A very large percentage of light single GA aircraft flying today were designed more than 50 years ago. :) The 150, 172, 182, and PA-28 were all designed in the 1950s, for example. Granted, I'd still agree that "safety precautions are minimal at best" to be inaccurate, though it would definitely seem that way when comparing them to the $100M-$400M passenger jets most people are accustomed to flying in.

As a teen I flew commuter flights in the mid 70's on propeller aircraft.  These were turboprop twins that seated a handful of people, although I don't recall the exact count, and I never knew the make and model.  I didn't find them scary, and my sister never seemed afraid.
They were annoying, however.  They were very loud.  The props were never synchronized exactly, so you had to listen to the beat frequencies generated by the heterodyning of the props.  I hate that.
I don't recall them being bumpier than the jets we also flew on when we were young, but it's possible that it was bumpier and just didn't bother us.
There was some compensation for the annoyance.  The pilots never closed the curtain behind them, so you always had a great view of approach and landing.  That was especially fun at night with all the runway and approach lighting.

Unless something has gone seriously wrong, planes are only “scary” to those who don’t know what “normal” feels and sounds like.
I’ve flown in everything from jumbo jets to light GA trainers, and they all make numerous motions and sounds that will be completely unfamiliar to someone who’s never flown before. Our brains are wired at a deep level to be scared of the unknown, because running away from unknown dangers is a better long-term survival strategy than trusting them not to kill you. But you can’t run away on a plane, which just magnifies that fear.
As you fly more, you learn that despite these feelings and sounds, you arrive safely at your destination, so they become “known” and less scary.  Your brain will even start anticipating them and take comfort in them because it means that things are going okay, and then it will start tuning them out. Eventually you will even start to wonder if something is wrong if you don’t experience those (formerly “scary”) motions or sounds when expected.
My high school band flew to a couple of national competitions, and many of the students had never flown before. They were paired with those of us who had, and I remember my seatmates waking me from my naps several times during each flight, panicked because they were certain we were going to die, and I had to reassure them that everything was fine before I could go back to sleep. Most were better on the second flight, and then a year or two later they’d be reassuring younger students themselves.

Although this site usually discards questions dealing with subjective matters, I'm glad to see this question has "survived" the peer review. My two cents:
Thing with "ye olde aircraft" is, that at their golden age they were the standard experience. Flying was supposed to be bumpy and noisy, and those flying frequently probably thought nothing of it. Throw a person from today back a few decades, and if not scared, they would be disappointed at least. The older planes would rattle and creak more, but so did other things in the daily lives of the people back then: cars, trains, ships, lawnmowers, buildings.
All in all, flying being scary has actually very little to do with what one flies on or with. Fear of flying as a psychological phenomenon is not rational. That's why it's referred to as flight phobia in scientific literature: a phobia is by definition an extreme or irrational fear of something, many definitions add a "disorder" to the description. I was unable to find quickly any studies about evolution of how people experience flying, but I would postulate that for a majority of people, it has not changed much over time.
It is worth mentioning that flying was more dangerous back then, statistics are very clear about this. From the seventies the fatalities in air travel have plummeted 12 fold from 6.35 to 0.51 fatal accidents / million flights (Wikipedia). So those embarking on a flight 50 years ago took a considerably larger risk than those taking a flight today.
More and more people can afford travel by air these days, that is a major factor when considering whether flying is more scary or not when comparing experiences over time. Planes are different, but so are the passengers as a group. Back in the days those flying were probably more adventurous, and flying carried a "high social stature". Might have been uncomfortable, but the bragging rights made up for it.
P.S. I personally never experienced older propeller aircraft (80's, so a bit outside the scope of this question) as any more scarier than the modern jets. As a kid, I flew several times on a regional propeller aircraft about the size of EMB 120 mentioned in a previous answer. I never found the ride to be scary, even though it was bumpier and noisier than what I now get to experience on contemporary and bigger jets. For me, it was exiting. A couple of years ago I was on a regional flight aboard an ATR-72, and the weather was bad. I mean really bad. Approach to home field was a real roller coaster ride with sharp bumps, long dips and high(ish) G soars. No doubt the cockpit was sprinkled with sweat after that. But was it scary? For me no, quite enjoyed it actually, as I trusted the plane and all the people involved in running the operation.

In the early 90s I had to fly on what I think was an Embraer EMB 120 Brasilia turboprop . As you can see, it's not a terribly large aircraft.
We were heading to Nashville and there was a major front moving in. It bounced that plane around worse than any flight I have ever been on. My poor and beleaguered mother was now terribly motion sick and the Brasilia had rather noisy landing gear, so when they deployed for landing, she grabbed my arm and begged me to tell her it was the landing gear. The pilot had already announced we were landing so I assured her it was. We landed without incident in the rain.
I never had anything similar in the ATR-72 aircraft that became common afterward. They were slightly larger, but I also suspect the airlines didn't fly them as readily into adverse conditions like that.
By contrast, the worst flight I ever had on a jet was on a 757. There was near-constant rippling of the wings, but nothing compared to that Nashville flight. The plane would "bounce" some, and I had serious vertigo the next day, but nothing serious.

I flew a lot of regional flights between the UK and Ireland in the 1990s, this was before Ryanair became what they are now, and the aircraft were often Short Brothers 360, which is a 36-seat twin prop. They were nicknamed 'The Vomit Comet' and for good reason.

Twin turboprops, so very noisy.
Unpressurised cabin, so uncomfortable ear equalisation problems for some people. Also this means a low ceiling meaning it could be difficult to go above bad weather.
Small, so tossed around very easily. As a passenger in one of these an approach into Knock Airport on the west coast of Ireland in the teeth of a North Atlantic winter storm is a very different proposition than it would be in even one of the smaller jets.

Hello Alan, Shorts Brothers are the world's oldest Aircraft manufacturer.


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Cessna 140 1946 TT
Hello I’m first time buyer airplane, I’m buying Cessna 140 1946 and it has TT 12.400
TSMOH  400  , needs anual  and ADSB
I’m I getting in trouble here ? Is too many hrs ? $17,000

Welcome to the site! If you are asking if you should buy the airplane we can't help you, as that is asking for opinions rather than fact based answers. If you have a specific question then I suggest you edit and ask it instead.

If this airplane has not been actively flown recently (and the lack of an annual and ADSB indicates not) then I would be cautious.  Has nothing to do w/being an old C-140 and everything to do w/neglected hangar queens.
Another question is will a C-140 bring you joy?  Have flown a two seat tail dragger?  They are more like kites, and a hot or windy day will be a challenge.
Good luck

The hours aren't really a problem.  Sitting around unused outside is the killer. These airplanes get ruined by corrosion long before they get worn out.  Of course you're going to have the airplane inspected by somebody who knows 140s as a condition of purchase right?
They are bulletproof generally, but there are a couple of areas to watch for, like cracks in the front door post wing attach etc (I believe there is an AD). You have to make sure the airplane is compliant with ADs.  And the engine is a big question mark. You can spend a fortune on the engine if you don't do your due diligence.
Did I mention that you should be having someone expert in 140s go over it for you right?  If you aren't and are just buying it as is, well get ready for possibly 10 or 20k of expenses if the engine is toast or the plane has corrosion.  Times and logs mean little.  What really counts is expert eyes going over it carefully.
If it checks out, 17k is an amazing deal for a solid nice flying 140.  They are delightful airplanes.


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Why do airspace shelves look like upside down wedding cakes?
Is it because the majority of traffic comes from higher altitude?

Those aren't upside-down wedding cakes.  Those are the iconic hats from the legendary band Devo that are safely stored in the heavens for the band's long overdue induction into the Rock and Roll Hall of Fame (hoping for 2023!).   https://www.youtube.com/watch?v=j_QLzthSkfM

Perhaps we should ask why wedding cakes look like upside down airspace shelves?

The purpose of airspace classes is to establish areas in which regulations are compatible with and appropriate for the type of traffic expected in that area and the ATC services provided for that traffic.
Let's look at Class B airspace, which is where you find the best-looking upside-down wedding cakes. The airspace around most major, high-traffic international airports is Class B. In this environment, lots of large transport-category aircraft are arriving and departing. As they arrive and get closer to the airport, naturally they have to be descending from cruise altitude through the arrival and approach. As they depart and get farther from the airport, naturally they have to climb up to cruise altitude. If you want to encapsulate this traffic in a chunk of airspace, the natural shape is an upside-down cone. To make it simpler to identify the edges of the cone at any given position, the shape is simplified to the upside-down wedding cake.
Why not just extend the airspace to the surface and have a cylinder? Because then you're imposing Class B regulations on all the little planes that are at relatively lower altitudes farther away from the major international airport, and larger planes arriving and departing from lower-traffic satellite airports in the area.
The same logic applies to Class C airspace, which also usually features shelves, albeit simpler and smaller ones.
The airspace designers pay close attention to the traffic flows and terrain around each airport when defining the shapes. For a cool example of this, check out Seattle's Class B:

All of Sea-Tac's runways (and approaches/departures) are oriented north-south, so there are "wedges" to the north and south that reach down to lower altitudes than to the east and west. The Cascade mountain range rises up to the east and southeast, requiring some special cutouts there too, and there are some more rectangular shelves to the east and west to accommodate "downwind" traffic. There's even a very small cutout immediately to the north to exclude traffic in the pattern at nearby KBFI (Boeing Field, officially King County International Airport). It's a great example of how the airspace is customized to fit the expected traffic flows.

Air traffic and instrument procedures need to be protected (be positively controlled airspace). Activity to/from an airport is diagonal (taking off from an airport and climbing, or descending to landing).
That means it's important to have the air near an airport protected, and the funnel (upside down wedding cake) covers the descending/climbing path without covering low lying areas that aren't needed.
Here's what the FAA says about design of the Class B airspace:

The Class B airspace design should be as simple as practical, with the number of sub-areas kept to a minimum. Its vertical and lateral limits must be designed to contain all instrument procedures at the primary airport(s) within Class B airspace.

The circular and stepped shape called a "wedding cake" appears in this description:

If a circular design is appropriate, the airspace may be configured in concentric circles to include a surface area and intermediate and outer shelf sub-areas... The altitude floors of sub-areas should step up with distance from the airport. Determination of sub-area floors should be predicated on instrument procedure climb/descent gradients to ensure containment of the procedures. Sub-area floors may be adjusted to have various floor altitudes considering terrain, adjacent regulatory airspace, and common vectored flight paths that are not on procedures.


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Why do the wings in these pictures go from swept back to parallel with the airflow?
To better understand my question, look at this A320 below

As you can see, the wing becomes parallel with the airflow at the wing root. Same thing on this 737

But this is not found on bigger planes like the 747 or A380. What is the propose of this?

You mean perpendicular to the airflow? Also, does this answer your question?

It does, thanks!


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Why would planes with two autopilots have just one active during cruise?
I know that the plane must have two autopilots for flying approaches, but why have individual buttons for each autopilot? Wouldn't it be easier to have both autopilots work at the same time for the entire flight?

It isn't necessarily to have both autopilots engaged to fly an approach unless that approach is below Category I minimums, i.e. lower than a 200' ceiling and 1800' of visibility (RVR). Down to Cat I, you can fly approaches with a single autopilot all day long, unless specific rules require something else.

It's usually one autopilot and two Flight Directors. The FD actually calculates the flight path and generates the commends on the primary flight display on its side, and the autopilot is just a dumb single channel machine doing what it's told by one of the FDs.  The buttons usually say FD 1 and 2, not AP 1 and 2. At least with conventional mechanical/hydraulic flight controls.  FBW is a whole different kettle of fish. That must be a panel from a plane with FBW?

@JohnK Yes, this is from the A320.

That makes sense. In that case the autopilot(s) are mostly software.

Just like real-life pilots, you only want one flying the airplane. The other is running in the background and can take control when selected. When you select which A/P is active, that just controls which A/P is actually flying the a/c.
Depending on the system architecture, how monitoring and fault detection are designed, the system can have one or more reversion modes that can be implemented. These can be automatic or require crew action, depending on the system and phase of flight. For example, in cruise you could fly on one A/P when the other is failed. But in approach, both A/P would need to be active to fly a coupled approach.

Actually many planes have 3 autopilots. One actually flies the plane and others check the one flying the plane. with 3 autopilots if the results from one, differs from the other two, it gets shut down and the plane continues with two. If there are only two autopilots and one gets different results from the other, both will shutdown and the pilot has to resume hand flying the plane.

In many aircraft, the self-monitoring and automatic reversion you describe only applies to autoland operations. In other phases of flight, including cruise, only one flight control computer is active and the pilots select which one. There is a very broad spectrum of sophistication across aircraft currently in service and the specifics vary from type to type.


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Will ADS-B eventually replace TCAS antennas on airliners?
I still seem to confuse myself about ADS-B. Will ADS-B eventually replace TCAS antennas on airliners? Or are those still a relevant part of the ADS-B system? And I guess just to add in, why don't smaller planes have TCAS antennas?

Yes. The future replacement to TCAS II is called ACAS XA. It uses ADS-B Out data along with the current TCAS II Mode S range and bearing to provide more accurate aircraft positioning, reducing the amount of false alerts and allowing better collision avoidance logic.
Initially, ACAS XA will utilize the same TCAS II antennas for backwards compatibility, however looking in future decades, removal of active TCAS II could conceivably take place.
Another version for is for general aviation, ACAS XP. It solely listens to ADS-B broadcasts and is in development. This doesn't work as well as full ACAS XA/TCAS II, but is much cheaper and intended for planes that aren't required to have TCAS II.

ADS-B OUT - Your transponder talks to a ground facility and is combined.with other aircraft transponders and retransmitted from the ground facility to those aircraft having ADS-B IN.
TCAS - Your system is talking with other TCAS systems around you, a 'conversation' between the two systems conveys position, altitude and direction of flight between the aircraft and determines, jointly, if there is a collision imminent, the two systems will issue CLIMB or DESCENT orders to both pilots in order to resolve the conflict.
The two systems are for two very different uses and ADS-B is not capable of performing the collision avoidance between aircraft. Therefore ADS-B will not replace TCAS II.

"retransmitted from the ground facility to those aircraft having ADS-B IN." That's TIS-B and ADS-R. TIS-B broadcasts non-ADS-B traffic, and ADS-R bridges 1090ES to UAT. In situations where everybody is using ADS-B Out on 1090ES, no ground infrastructure is used. One example is in-trail procedures in oceanic airspace, where aircraft use ADS-B In to follow other aircraft at reduced sepration.

Would this mean that a TCAS equipped 737 would not warn the pilots of a ADS-B equipped Cessna 172?


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Buzz-saw noise from big turbofans at high thrust
This topic has been addressed many times on this and other forums. I have seen (and often not fully understood) some deep technical analyses, incuding manufacturers' research labs, since this is a significant component of take-off noise. (-And all that extra mechanical energy might have adverse effects on blade and liner life.)
Answers generally seem to include the notion of fan tips going supersonic, sometimes with complications vie reflections from stator structures. But the origin of the specific low-audio frequency is not addressed anywhere I've seen. How does a many-bladed fan spinning at a few thousand RPM generate a (harmonic-rich) fundamental around 50 Hz?
In the past few years, I've submitted a possible theory to a couple of other forums, but never had it accepted by moderators. I'm hoping that this group might be helpful.
Thinking about potential acoustic resonances in a turbofan engine, one presumably wants to find the structural element of greatest size in at least one dimension. The core might be considered to be as long as the whole engine, but there are many interruptions in the flow path. The bypass casing, however, might make a nice organ pipe at about the right frequency, excited either asynchronously or as a deep submultiple of the rotating blade-shock frequency.
One phenomenon that this might explain is the slight drop in bass note when power is reduced after takeoff. Fan and core shafts presumably slow down very quickly. But if the main sound is from a pipe resonance, that shouldn't be affected. However, reducing RPM also reduces compression in the bypass channel. This presumably reduces temperature, hence speed of sound, making the pipe acoustically longer and lowering resonant frequency.
Is this silly, or perhaps obvious? The idea may appear elsewhere, but I haven't found it.
Thanks for your thoughts.

What I'm missing is why the buzzing sound from such a symmetrical object as a tightly bladed fan should be one-per-revolution.

It's not.  "Buzz saw noise" as I would define it, has a fundamental frequency at the fan blade passing frequency.  i.e. if there are 20 fan blades, then "buzz saw noise" has a fundamental at 20/rev.  As you mentioned, this noise is mostly due to the shock wave generated as the tips of the fan blades go transsonic.
Now there is noise generated by an engine at 1/rev, but it has nothing to do with the fan blades.  This noise is generated by the unbalance of the rotor.  Because the rotor is never perfectly balanced, there is a force between the rotor and the bearings at 1/rev.  This force gets transmitted structurally up from the bearings, through the frames and cases, through the mounts, through the wing, and eventually into the fuselage. Throughout this whole path, it's not noise, its a structural vibration.  It can become acoustic noise when the fuselage motion starts to cause the air inside the cabin to start vibrating.  But you can also just feel it.  In programs I worked on, the airframers would put accelerometers on the seat tracks to determine how much 1/rev vibration the passengers would feel in their seats.

If it is due to rotor imbalance, shouldn't the sound be unique for every engine? but what we observe is that it is always same for a particular engine type, and different for different engines. Also, why is this sound only heard at those speeds at which the blade tips are close to M=1, and not above and below that narrow RPM range?

Well, the amplitude of vibration will certainly vary from engine to engine, depending on how well the rotor is balanced.  But the frequency will be basically the same within a given type.  Between types the frequency could be significantly different.

Fair enough; but what about my other question? why is that sound heard in such a narrow RPM range (and not above and below that range), and why does that range coincide with the RPM range at which the blade tips are close to M=1? (here is an example of a GE90 that depicts what I'm talking about)

Also, the only engines that I know of which do not produce a "buzzsaw" are PW GTF engines (other than PW1100). PW GTFs also happen to be the only turbofans (atleast those that I know of) whose blade tips do not exceed the sound barrier. Is that a coincidence? and so are the rotors of PW GTFs perfectly balanced?

if there are 20 fan blades, then "buzz saw noise" has a fundamental at 20/rev  Hmm, if the blades are identical, a blade (that sounds like the same blade even though it isnt) goes by at 20 TIMES rev.  Only if the fan has a significant assymetry would you hear the fundamental at the RPM.  Note that car tires used to have equi-sized tread blocks and make a high pitch at wheel RPM * treads per tire.  Now, if you look closely, you'll see the blocks are all different sizes, so nowadays tire pitch is down at wheel RPM, much less noticeable.  (I don't know airplanes but know audio engineering.)

"and why does that range coincide with the RPM range at which the blade tips are close to M=1?" Well the unbalance forces scaled with speed squared.  So at idle ~20% N1, the unbalance forces are only 20% ^2 = 4% of the forces at takeoff ~100%.  Forces can be amplified by resonances, which is a whole other topic, but generally unbalance forces are highest at high speed.  Blades are also designed to be transsonic at high speed.  So it is a coincidence that both sources of noise are highest at high speed.

The only engines which do not produce buzzsaw also happen to be the only engines whose blade tips do not exceed M=1. Is that a coincidence? and are those engines (PW GTFs) perfectly balanced? or is there a relation between buzzsaw and the blade tips reaching M=1?

@AdityaSharma no that is not a coincidence.  Buzzsaw noise is generated when the blades tip are close to mach 1.  PW GTFs are definitely not perfectly balanced.  Some are better than others, but none of them are perfect.

re 'Buzzsaw noise is generated when the blades tip are close to mach 1." In your answer, you say that buzzsaw is generated due to rotor imbalance. Which one is correct?

I did NOT say that buzzsaw noise is generated by rotor unbalance.  I said that there are TWO different sources of noise.  One is due to rotor unbalance and one is due to fan blade tips hitting Mach 1.  They are two completely different things.

I am not sure what "buzzsaw" you are talking about. This, this and this are examples of the buzzsaw that I'm talking about. All three of them are less than 100Hz (approximately 1/rev). Is that "buzzsaw" sound caused by rotor imbalance or by the blade tips going supersonic? Also, if possible, please share a video of the "buzzsaw" that you are talking about.

How does a many-bladed fan spinning at a few thousand RPM generate a (harmonic-rich) fundamental around 50 Hz?

According to its certification documentation, the Pratt&Whitney PW1100G has a low-pressure rotating speed $N_1$ of 10'047rpm. This low-pressure stage is connected to the fan via a 1:3.0625 gearbox, therefore giving a fan speed of 10'047/3.0625=3'280rpm. This translate into some 3'280/60=55rps (revolutions per second) aka Hz. Now, due to the impossibility of building a perfectly balanced rotating machine (even if the blades might have a difference of only a couple of milligrammes), the rotor vibrates at this particular frequency therefore generating the relevant noise which gets to the cabin through the air; plus, since the turbofan is connected to the airframe, there's also a structural path letting these vibrations travel till the cabin.
So, even without taking into account any aerodynamis phenomenon, only from a pure mechanical point of view the fundamental "noise" frequency of a modern jetliner's turbofan is just in the range that you have expected.
Obviously for the low-pressure stage the fundamental frequency is 3.0625 times as much i.e. some 170Hz. And for the high-pressure stage rotating at 22'300rpm, the fundamental frequency is 22'300/60=370Hz.
At all these fundamental frequencies we have to add their whole multiples and especially the frequencies given by the fundamentals times the number of blades: for the fan for example the frequency 55x19=1kHz should also be important.
And then there are all the noises having an aerodynamics source but, as you pointed out, that's should have been already addressed by other answers on this site.
As a side note it's nice to know that the whole airplane's structure and all the components installed on the aircraft should be designed and tested so that they have no  eigenfrequency close to those fundamental frequencies in order to avoid possibly catastrophic resonance phenomena.

Thanks for that, sophit. What I'm missing is why the buzzing sound from such a symmetrical object as a tightly bladed fan should be one-per-revolution. My impression from propeller-noise analysis is that interference with nearby structures makes much of the noise. If the sound mostly radiates forward, with a wavelength larger than the engine, why don't all the single-blade sounds cancel? Or perhaps they mostly do, and thae buzz is just a small residue of the local SPLs?

@cTen: from a pure mechanical point of view I'd expect the main noise frequencies to be the ones due to the rotating speeds, so 55Hz for the fan, 3.0625 times as much for the low-pressure stage and so on. Obviously plus the frequencies multiple of the fundamentals. And plus the frequencies given by the fundamentals times the number of blades: for the fan for example I'd expect the 55x19=1kHz to be also important. And then there are all the noises having an aerodynamics source but that's a much more complicated matter.

Knowing the throat length of the inlet duct from the lip to the compressor disc face, one can calculate the "organ pipe" resonance mode of the air entrained in there and see if it coincides with any of the blade passage modes. This would turn the inlet duct into a resonator.

avoid possibly catastrophic resonance phenomena -- yes, good safety tip!  :-D

Above frequency spectrum plot, earlier used in this answer, was cc-d from a now defunct link and shows fundamentals of 75 Hz, way below the blade passing frequency. @DanielK points out that the 75 Hz fundamentals coincide with the vibrations of the fan rotor unbalance, amplified by the fuselage:

This vibration energy is transmitted as structural vibration through the engine structure, through the pylon and the wing, and then causes the fuselage itself to vibrate at that frequency. This causes the interior of the cabin to become a giagantic loudspeaker, at the rotor frequency.

The fan unbalance loads on the structure are a function of thrust provided by the fan, so reduction of power after TO reduces both the fan frequency and -amplitude.

vibrations of the fan rotor unbalance, amplified by the fuselage -- fuselage wouldn't "amplify" but it could resonate.

@SwissFrank Amplified, like the soundboard of a classical guitar.

I'm sorry but that's not amplification either.  If you have a source that says it's amplification it is wrong.

...specifically, an amplifier adds power to a signal. The sounding board/box of a stringed musical instrument couples the power of the vibrating strings to the surrounding air, but it does not add any power.

You may look up the definition of the word amplify. It is not congruent with adding power to a signal. The gist of the original quote was: the sound is made more audible.

I followed your link. I didn't see any place where that page talked about making a sound more audible. I did see definitions about "increasing the strength" of something or, making something "larger or greater."

How does a many-bladed fan spinning at a few thousand RPM generate a (harmonic-rich) fundamental around 50 Hz?

Easy, but you'll need to bear with me :)
For the electrical grid to function, the loads on the grid must all be met by standing up generating capacity - or you get into annoyances like rolling blackouts, or uncontrolled grid collapse.
However, loads on the grid are highly variable through time - peaks occurring a few hours a day, being different on weekdays and seasonally. In summer, load at 6 pm can be 3 times load at 2AM (we're talking pre-electric cars; those may really help fill in usage valleys.)
Why are we talking about electricity?
To cover peaking loads, they have peaking units aka "peakers".  The most popular peaker is an airliner jet engine core turning a generator instead of a fan, and run on methane instead of Jet A in most cases.  These "aero-derivative" units are lightweight, movable and can be serviced by the aviation supply chain.  (notably, 40 years ago, peakers were locomotive engines, which are optimized for 1000 RPM = 50Hz / 3.)  For instance

Pratt & Whitney GG4/FT4 (24 MW) <- J75/JT4 - Delta Dart, F105, B707, DC-8
Pratt & Whitney GG8/FT8 (25-30 MW) <- JT8D - A6, A4, B727, B737 original, DC-9
GE LM2500 (20 MW) <- TF39 (early CF6) - early C-5 Galaxy
GE LM6000 (40-56 MW) <- CF6-80 - B767, A310, 747-400, MD-11, C-5M
GE LM9000 (66 MW) <- GE90 Boeing 777

Some of these had longer careers as stationery/marine engines than as aircraft engines! GE will still sell you a new LM2500.
Mind you, it's not like they design for peaking only and then try to fit it to aviation as an afterthought. But it's not the other way round, either.  If they feel that engine fits the stationary generator market, the peaking application is on their mind from day one. Note that 1500 and 1800 RPM are also useful for generation since 4-pole generators can be used.

Almost looks like a scene at an FBO.
The biggest markets for peaking units use European style 50 Hz grid frequency.  As such, jet engine designers have incentive to design the N1 shaft speed to have 3000 RPM (50 Hz) as a "sweet spot".  Pretty much simple as that.
Now they do make heavier-built turbine engines, probably marine derivative, that have better long-term durability - the tradeoff being more costly, and too heavy to easily ship offsite for rebuilds, chaining you to the local labor supply and skill levels.  A lot of peaking plants run only a few hundred hours per year and such engines would be costly overkill.  If you're thinking "It's a darn shame to have so much costly infrastructure that is so rarely used", I'd agree.  And if you're thinking "Wait, electric cars and home batteries could be a game-changer", I agree too.  But it's easier than that; we just haven't gotten organized.

Comments are not for extended discussion; this conversation has been moved to chat.

I have a woodfire burner in my sitting room with a 25 foot long 5" diameter flue pipe. When lighting this with a firelighter block, the flue pipe will start to resonate so strongly like a 25' long organ pipe that it shakes the whole house (solidly built, all brick farmhouse dating from 1837), One can control the volume of this resonance by varying the draft to the fire but the frequency remains pretty constant and as the draft is reduced, the resonance will suddenly cut off completely. I can therefore well believe that the low frequency droning on high thrust (CFM 56 engines and their close relatives seem the most prone to this) is due to organ pipe type resonance of the tube that forms the channel for the air from the ducted fan. The exciting factor for this may be variations in the air flow from the fan but the basic frequency would be a function of the length of the tube and the intensity controlled by the volume of air passing through the tube.

Your answer could be improved with additional supporting information. Please [edit] to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers in the help center.


text:
Which aluminium alloy is used in the Boeing 747's fuselage?
Which aluminium alloy is used in the Boeing 747's fuselage?

I will, Thanks.

Related

The primary alloy for the original 747 is 2024-T3, which is not the strongest or otherwise perfect, but a well-established convenient choice. Second is 7075-T6, used for major load-bearing parts.
7075 is stronger, but less workable and less tough than most; it comes closer to the edge for aluminum, so to speak.
6061 follows for for less-loaded or some more complex parts, where it's a staple, being overall the easiest to use out of common Al alloys. This is not nearly an exhaustive list, just the main alloys.

Multiple alloys are used. Here is a recycling composition breakdown of this aircraft, not specific to the fuselage:

Materials of Civilian Aircraft


text:
How are the runways operated at Toronto Pearson?
I have a sort of technical question regarding the operations of runway systems at larger airports, specifically at Toronto Pearson (satellite view here). As far as I can see, the airport has 5 runways: 05, 15L/R, and 06L/R.

(Public domain via Wikimedia Commons)
Since RW05 and RW15L/R intersect it is clearly infeasible to operate RW05 at the same time as RW15L / RW15R. As for the rest, there are several ICAO guidelines regarding parallel or near-parallel runways. What do those restrictions imply regarding simultaneous operations?

Is it possible to operate RW06L and RW06R simultaneously, maybe in a segregated mode of operation? What about RW15L/R?
Similarly, can runways 15 and 06 be operated simultaneously?

Thank you for specifying Toronto Pearson, which is distinct from Pearson (KVUO).

"Since RW05 and RW15L/R intersect it is clearly infeasible to operate RW05 at the same time as RW15L / RW15R." This is not clear to me. Obviously at the same exact moment in time you can only use one or the other of the intersecting runways, but that is not to say that using one removes the other from consideration. The controller simply has to ensure proper intersecting runway separation.

I don't separation distances between the runways, but 06L & 06R are pretty close, so I imagine they could only do staggered parallel use there.  15L & 15R are also pretty close, so would also limit them to staggered parallel use.
05 and 06L look far enough apart for simultaneous operation. In the US the requires distance is 4300 ft to give vortices a chance to dissipate and not affect a plane operating in parallel. I don't know if Canada has the same requirement.

Is that for simultaneous parallel?  The 2006 paper I cited in the SFO link says  "ICAO and FAA have accepted closely spaced parallel approaches with the PRM as meeting the TLS.
Extensive demonstrations and tests, consultations among the concerned parties, and sound judgments by operations experts finally led to reductions in the required spacing between parallel runways. The required spacing has now reached 3400 feet for parallel approaches, or 3000 feet if one of the approaches is offset by 2.5 degrees."

@ymb1 SFO and similar only allow simultaneous operation on parallel runways that closely spaced in good weather. Fog at SFO cuts usage to a single runway, the source of regular horrendous delays there.


text:
What is the difference between a Stand and Gate in Airports? How do they differ ?
I have seen Aircrafts having been assigned a Stand and Gate, So wanted to know whether the stand and gate are same physical location of airport or they are different but have link between them.

An aircraft stand is a place where an aircraft can be parked.
A gate is a designated area inside the terminal building where passengers will assemble and wait prior to boarding. It includes facilities for checking boarding passes, last-minute customer service such as seating changes and so forth.
In the typical setup most passengers meet, each gate serves just one stand located right outside it. But this is not always the case: airports often have remote stands too, and there will be a smaller number of gates from where passengers are bused (or walk across the apron!) to the appropriate stands. Such a gate is not dedicated to a particular aircraft stand.
In the former case, where a stand is served by a dedicated gate, there is little point in distinguishing between the two. They will often (but not always) share one identifying number, and people happily speak about, for example, taxiing to the gate after landing. But for remote stands a distinction needs to be made.

In addition, on some airports, a single stand may serve two different gate numbers. Amsterdam shares the same stand for Schengen and non-Schengen, but two different gates on two different levels inside the terminal building.

@vidarlo: Good point. There are a handful of such stands in Copenhagen too.


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Where can I find the ownership of a specific aircraft registered in the UAE?
Is there a website where I can find the ownership of a specific aircraft - or in my case a specific corporate jet - registered in the UAE? I am conducting some research on corporate jets and need some detailed information about the ownership of a particular aircraft.

mabye the plane spotters have "caught" it: https://www.planespotters.net/

i checked planespotters.net and few other sources and got all the information about the aircraft itself including its MSN, production site, engine type and registration number but no information on its ownership. It is a A319 corporate jet registered in UAE, which makes it a bit more difficult to find out about the ownership due to strict UAE laws. Was hoping someone more knowledgeable than me might be able to help.

Try this site as well http://www.landings.com and select Databases


text:
Why does an aircraft fly a looping when pulling the stick?
let's assume an aircraft flies in straight level flight in the direction of an x-axis with an angle of attack of 3°. When the pilot pulls the flight stick, an additional lift is produced which acts perpendicular to incoming air direction. But in order to fly a looping, the lift force should always be perpendicular to the body axis/attitude of the aircraft?
So the thing is, to my understanding when we increase the angle of attack, the lift force increases (drag also increases a little bit). This should only mean that the aircraft climbs now, but the attitude or to be precise: the aircraft flies still into the x-direction but has now also an y-component (altitude). But there must happen something with the forces so that a pilot can fly a turn.
Can you follow me?
Thanks for your help in this!
Lucas

You forgot to consider what directly happens when you pull the stick (which is not producing additional lift with the wing)

Dou you mean: When I pull the stick then "directly" the tail goes down and the nose up.

Consider: looping always involves pulling the stick back (well, could think of a few exotic exceptions such as a very rapid roll to wings-level from a steep fast spiral dive, but that's the general rule), but pulling the stick back doesn't always make a loop.  Hmmmm....

Yep,  your tail goes down (i.e. the nose up) and you gain incidence. Afterwards it's just a matter of equilibrating the forces acting upon the aircraft.

In any turn, forces do not equilibrate.  As you pull the stick back, your nose changes direction.  A loop is a turn in the vertical.  As you turn, the wing is constantly pulling in a new direction.  As @quietflyer  said, the physics are similar to a level turn, but you need enough energy to turn past 90 degrees vertical, of the plane will fall backwards.  In considering what an elevator  control input does, it is a rotational torque about the pitch axis and a change in direction of the plane.

@sophit overall, forces do equilibrate: more drag in the old direction, more lift in the new.  Throttle is increased into the turn (to hold altitude), but is generally constant in the turn, proving you are correct as well.

@RobertDiGiovanni: I know. What I mean is that pulling the stick is not the end of the story, it's just the beginning. And for sure you don't keep it pulled (unless you are Tom Cruise in Top Gun)

In the loop, as the aircraft goes to vertical, the gravity vector moves behind the aircraft, slowing it rapidly.  Throttle must be increased to maintain velocity.  But the wing is still "turning" the aircraft.  The loss of speed results in an "egg shaped" loop, as the process reverses on the way down.

@Lucas: let me know if you've understood the steps how it works, otherwise I can come up with an answer.

@RobertDiGiovanni -- a sailplane can fly a nice round loop, if the G-load (statement can also be expressed as control stick displacement, control stick force, angle-of-attack, pitch rotation rate, etc, insert other parameter of your choice here) is applied correctly.  E.g., certainly not just a constant 4-G pull-- which I was actually taught once in real-life dual instruction, as a method for a crude, beginner-level, not round (likely egg-shaped) loop!

@quietflyer yes, an experienced pilot would have speed control down to carve a nice circle.  First time through I'd probably "egg" it once I got to 90 degrees.

You might as well ask why an airplane doesn't just start sliding sideways, rather than flying in a horizontal circle, when it banks (while increasing lift as needed so that the flight path remains in the same horizontal plane.)  The two situations have a lot in common.  The key attribute of a circling flight path, be it a horizontal or a vertical circle, is that there is constantly a net centripetal force acting perpendicular to the instantaneous direction of the flight path.  This net centripetal force makes the flight path curve, or change direction.  Meanwhile the aircraft has an inherent tendency to stay generally aligned with the instantaneous direction of the flight path, albeit with some offset angle.  (Oversimplifying a bit, we call the offset angle the "sideslip angle" in the case of a horizontal turn, and it tends to stay near zero in most aircraft, while in the case of a vertical turn we call the offset angle the "angle of attack".)
By the way, keep in mind that in a linear climb, as opposed to during the initial pull-up or during the start of a loop, lift is less than aircraft weight, not more.  For more see related ASE answer Does lift equal weight in a climb?.
Keep in mind too that though you are right to point out that the lift vector is defined to act perpendicular to the flight path, not the longitudinal axis of the aircraft, it would be possible to re-define the lift vector so that it was tied to the "body frame" of the aircraft, but that wouldn't fundamentally change our view of what is happening during a loop.
You may find it helpful to first think through the problem with the simplifying assumption that the aircraft's rotational moment of inertia about the pitch axis is zero.  This basically means that the aircraft is going to instantly assume whatever angle-of-attack the pilot is "setting" with the control stick, which is fundamentally an angle-of-attack controller.  You can seen that it is impossible for the flight path to stay linear if the net force is not zero, i.e. if there is a net centripetal force on the aircraft.  Once you are thoroughly comfortable with this concept, then you may wish to also consider how the aircraft's actual non-zero rotational moment of inertia about the pitch axis might slightly complicate the stick inputs required of the pilot in various maneuvers.

TL;DR it may be useful to consider what happens when the aircraft has rotated 90 degrees and is pointing upwards.  Perhaps the OP considers that it will be ‘stuck’ here and won’t rotate further, although it should be obvious that the control surfaces and relative airflow rotate more or less with the aircraft.


text:
PTT mutes mic w/ A20, Delta Zulu, works with older headsets
New airplane to me. Coms work great using an older headset, David Clark or other brand.  When using my Delta Zulu or Bose A20 the intercoms works fine until you press the PTT.  When pressed the radio keys up but the mic is dead, no sidetone, no audio transmitted from the radio. Thoughts on what could cause this issue only with the newer headsets?  Thanks for any help

A similar problem with the Bose A20:  https://aviation.stackexchange.com/questions/64522/instructor-cant-talk-when-h10-13-4-is-plugged-in/64633#64633

The head set works fine with the intercom, good sidetone and I can talk to other passengers. The problem is only when I key up the radio that the problem occurs.  This is with the pilot side using a yoke mounted inline PTT or the passenger size using the dash mounted PTT.  It’s like the PTT ground connection mutes the mic?


text:
Would laminar flow airfoils work for a canard aircraft?
The L/D benefits of laminar airfoils seem hard to pass if flying within their Reynolds number. But what would happen if you used them for canard aircraft? Could they be used on an aircraft like the Rutan Quickie or would it not be wise to use them for a main lifting surface because of the risk of stall...

Well designed laminar airfoils have very benign stall characteristics. They are designed to not stall even when transition is happening very early. This can be seen in their lift curve slope which has two peaks, the first and lower one for mostly laminar flow, and the second at a higher angle of attack and lift coefficient, with a fully turbulent upper side. This second peak is shallow and wide, so lift will not drop off a cliff as it does in 5-digit NACA airfoils when going beyond the angle of maximum lift coefficient.
Two things need to be considerd when designing a canard with laminar airfoils:

Wing sweep should be kept low. Sweep will cause a rotation of the local speed vector within the boundary layer which encourages early transition. With sweep below 20° or better 15°, laminar airfoils should work well. Less sweep will also allow to use a higher aspect ratio in order to stay below the sweep - aspect ratio limit for poor wing stall characteristics.

Make sure that the wake from the canard will stay above or below the wing. The turbulence in the wake will destroy laminar flow. Of course you can design your canard such that its wake will start to hit the wing as the configuration approaches stall. But this will affect overall lift much more as when the wing wake hits the tail in a conventional design (a rather poular natural stall warning) because the main wing is so much larger than the canard.

I helped a friend build a Q2 many years ago. One thing we learned was that rain had a serious negative affect on the laminar flow. Even a light rain would cause significant loss of lift. While he had no options, I don't know how you could account for it in the design.

@Gerry That was a common problem with laminar airfoils from the Sixties and Seventies. Everyone who has ever flown a Pik-20 or a Janus A in rain will agree. Those airfoils reached their maximum lift coefficient only when clean. Modern designs achieve their maximum lift coefficient with a fully turbulent upper side and fly almost as well in rain - turbulence will lower L/D, but not affect minimum speed.

Oh I see thank you. You can see the two peaks in the NLF airfoil lift curve., that's interesting. The GU airfoil drop dramatically, more so than the LS1.

Hi @PeterKämpf I have a design for a VTOL aircraft I am designing based on the Quickie. Do you still do any consulting? If possible please let me know!! Kind regards

@JoshCarr Never seriously did consulting, but keep asking!


text:
Which European (EASA, EU) regulation corresponds to US FAR 91.13 on "careless or reckless" operation of aircraft?
The US has 14 CFR 91.13 about "careless or reckless" operation of aircraft, as a sort of catch-all for activities which are not explicitly prohibited by the regulations but which are still a bad idea and risks to, or actually, puts others in danger:

§ 91.13 Careless or reckless operation.
(a) Aircraft operations for the purpose of air navigation. No person may operate an aircraft in a careless or reckless manner so as to endanger the life or property of another.
(b) Aircraft operations other than for the purpose of air navigation. No person may operate an aircraft, other than for the purpose of air navigation, on any part of the surface of an airport used by aircraft for air commerce (including areas used by those aircraft for receiving or discharging persons or cargo), in a careless or reckless manner so as to endanger the life or property of another.

I have a hard time imagining that Europe doesn't have a similar statute, but I can't seem to find anything looking through Part-FCL or my local regulations.
What is the EASA or EU regulation corresponding to the US FAR 91.13?

A good question. Keep in mind every EASA member country has its on variation on aviation regs, but they should all be roughly similar.

This is covered by SERA.3101 (Page 19) for the UK:

SERA.3101 Negligent or Reckless Operation of Aircraft
  An aircraft shall not be operated in a negligent or reckless manner so as to endanger life or property of others.


text:
Under EASA regulations, what constitutes loggable flight time?
It wasn't too hard to find questions and answers here discussing flight time as defined by the US FAA.
However, I'm curious about EASA regulations, and if applicable specifically as implemented in Sweden.
Based on the relevant EASA regulations, at what point do I note the start and end time of a flight for the purposes of flight time logging?
I looked through the copy of the regulations that I have, but couldn't find anything relating to logging (of flight time or otherwise).
Citations, please!

According to FCL.050(g):

Flight time is recorded: (1) for aeroplanes, touring motor gliders and
  powered-lift aircraft, from the  moment an aircraft first moves to
  taking off until the moment it finally  comes to rest at the end of
  the flight;

That seems to be the same as the FAA regulation.


text:
Is contact between flow and inner walls of nozzle required for thrust generation?
I uploaded a picture below explaining over-expanded flow/nozzle condition with regards to thrust.
This aviation source tells that “extra nozzle walls don’t generate any additional thrust because flow detaches from inner walls of nozzle (in other words, flow losses contact to inner walls of nozzle)”.
Does it mean that velocity of flow when flow pressure matches ambient pressure a distance back from nozzle exit must be taken for thrust equation? In general, we calculate thrust based on exit velocity but here why don’t we do it?
I asked Artifical Intelligence this question and it says: “Even after flow pressure matches ambient pressure, flow velocity still increases resulting in thrust increase. So, we should still take exit velocity, not velocity when flow pressure matches ambient pressure”.
I’m confused. What is your answer?

It may be (with further studies) things will become more clear.  Remember, that flow acceleration from the combustion chamber is greatest at the "throat" of the exit from the chamber, not the end of the nozzle.  The "bell" is a pressure recovery device.  Your rocket will still have thrust without one, but Isp will be lower.

Quite sure you'll get a better answer on Space Stackexchange

Yea, and i wonder if there may be a second (aerodynamic) "throat" , so maybe (theoretically) the flow could expand until it is subsonic.  All that balanced against weight.

The flow exerts force on the engine by pressurizing the walls. If the flow detaches from the wall, it cannot exert force on the engine bell. Instead the atmosphere exerts pressure on this section, resulting in net 0 forrward force. The extra nozzle wall is unnecessary weight, therefore decreasing efficiency.

Does it mean that velocity of flow when flow pressure matches ambient pressure a distance back from nozzle exit must be taken for thrust equation?

Yes, as it does not accelerate anymore.

Is contact between flow and inner walls of nozzle required for thrust generation?

In this sense, yes.
However note that the reason for the detachment is overextension of the flow.

On a direct physical level, thrust is produced in the nozzle through higher-than-ambient pressure acting on nozzle walls from the inside. Ambient pressure acts on both sides of the surface equally.
Separated flow produces no or little thrust or lift. This is how airplanes stall - when flow separates from their wings.
In rocketry, it's common to describe propulsive efficiency through exhaust gas velocity. It works, but Newtons' third law only applies to the interacting parts of the system. Eventually, any rocket's exhaust will slow down to zero - that doesn't make their Isp zero!
This is also confirmed by practice. Nozzles designed for vacuum are larger than atmospheric nozzles. The same rocket always produces more thrust in vacuum than in the atmosphere.
P.S.: Do NOT ask Large Language Models questions that require deep technical knowledge. They don't have it. It's like a young kid that has read a lot of books - they can occasionally cite true facts, but as easily cite or invent fiction, and lack the wisdom to tell one from the other.

So do you mean that thrust is generated until flow pressure matches ambient pressure a distance back from nozzle exit, and there is no thrust produced after flow detaches from nozzle walls resulting in loss of contact?

It depends on how you model things. But the physical interaction that produces the thrust force depends on a pressure difference between the inside and the outside of the nozzle.

Essentially, thrust comes from above-ambient pressure, and flow separation occurs when the exhaust gas is below ambient.  Incidentally, this is something to avoid as it tends to result in severe vibration and can damage the bell.  From a thrust point of view though, separation can be seen as a good thing because the lower-than-ambient gas pressure would actually reduce thrust.

There is one obvious fact about rockets.  They are generally (but not always) made for vertical flight.  In other words, they launch at ground  level pressure and fly to space.

with a vertical flight path, if flow is overexpanded at launch, it won't be for long.

Rockets are heaviest at launch. Best to have maximum nozzle efficiency here, and underexpand when the rocket is lighter.

staging can allow for more suitable "vacuum nozzles" at higher altitudes

Remember, a nozzle is designed to improve propulsive efficiency.  This must be balanced with the weight penalty of a larger bell.


text:
How can over-expanded flow go out of nozzle when it has less pressure than ambient pressure?
We know that over expansion in flow means that nozzle exit pressure of flow is lower than ambient pressure at specific altitude. It gets narrower while exiting nozzle causing loss in efficiency.
My question is;
How can over-expanded flow with less pressure than ambient pressure go out of nozzle and go into ambient pressure which has more pressure than it?
It sounds weird to me because in this case, flow go towards high pressure zone from low.
We might say flow gained kinetic energy as well as momentum before in combustion chamber, but for it to move towards nozzle and turbine, it must still have more pressure than ambient pressure, so that it can move.
I am confused to be honest.
Can you help?

As soon as the flow reaches the ambient pressure it detaches from the nozzle and it just continues "straight on" without following the nozzle contour anymore. But let's wait for an answer from some expert here.

Good reading here.

@sophit, actually the wikipedia page says it only actually detaches when overexpanded by about 40% (until then it will just slow back down a bit to match the ambient pressure).